The forward link (base station-to-mobile subscriber) of an IS-95-compatible CDMA cellular telephone system is subject to a variety of quality of service (QoS), range of coverage and traffic capacity limitations stemming from insufficient transmitter power. A 100-W transmitter may yield as little as 8 W at the antenna terminals after line losses and as much as 9 dB power backoff to accommodate peak-to-average power ratio of the non-constant envelope signal. Since there typically is no diversity on a forward link, a stationary or low-velocity mobile in a heavy multipath environment (e.g. urban canyon) can experience long fades not correctable by coding and interleaving, the result of which typically is a dropped call. A second problem is that reverse link capacity is often not employed most beneficially, since a mobile may not have access to the strongest available reverse link.
All proposed solutions to these problems require application of additional transmit power. To preserve the link to a user in a deep, slow fade, both of the following have been suggested: (1) command a substantial power boost on that user's forward link; or (2) employ dual-diversity forward link transmission. Reverse link efficiency can be enhanced by putting the user's forward link into hand-off mode with respect to multiple base stations, thus offering the mobile subscriber a choice of reverse links. The extent to which any of these remedies succeeds is limited by availability of transmit power.
Similar circumstances may result if the transmitter is a satellite or other facility.
The object of the present invention is to provide forward link multiplexing and method to alleviate these limitations. The notion is simply to use the available power more efficiently. Rather than replace the high power amplifier (HPA) to increase the transmitter power, one replaces the multiplexer.
Linear superposition of chip-synchronous, orthogonal signals (as in the IS-95 forward link) is a theoretically lossless multiplex if the subsequent transmission chain remains linear. Maintaining linearity requires a linear HPA. Since any HPA characteristic eventually saturates as its input power grows (see FIG. 1), IS-95 base station transceiver linear amplifiers are typically run at 4–5 dB average power backoff to accommodate peak power needs. (Third generation wireless mobile networks, e.g. cdma2000, might serve a greater number of subscribers per base station and require correspondingly greater backoff.) In addition, the rather severe spectral containment filtering applied to each user signal before multiplexing creates amplitude fluctuations of 4–5 dB peak-to-average power, requiring additional backoff. Thus total backoff can easily be 9 or 10 dB.
An alternative approach to producing greater average power is to achieve a more effective allocation of the loss budget between the multiplexer and the HPA. Applied to orthogonal waveforms, non-linear multiplex methods that produce a constant envelope signal will permit a greater fraction of the available transmitter power to be used for communication, but at the expense of a multiplexing loss that may be characterized as either cross-talk (induced non-orthogonality or harmonic distortion) or receiver cross-correlation mismatch. This multiplexing loss, however, is typically smaller than the power backoff it replaces, resulting in a favorable trade.
The manner in which a CDMA base station processes input user data to create a baseband signal that is a multiplexed composite of multiple CDMA codes is illustrated in FIG. 2. The multiplexing indicated in FIG. 2 is fully additive, or linear. Each user data stream or channel 10-1, 10-2 . . . 10-N is rate-½ convolutionally encoded 11 and the encoded symbols are repeated (as a function of data rate) to produce a 19.2 kb/s stream. These symbols are interleaved and then covered (scrambled) 12 with a PN sequence. The resultant is then modulated 13 by a repeating 64-bit Walsh word that identifies the channel; chips of the Walsh word are at the system chip rate of 1.2288 MHz. The signal is then split into two paths for quadrature spread spectrum modulation; after separate I-channel 14 and Q-channel 15 codes (called the pilot channel PN sequences) are applied, both signals are baseband filtered 16, 17 to retain only their central spectral lobe for spectrum control. This step introduces amplitude fluctuations with a peak-to-average power ratio of 4–5 dB. Multiplexing 18 then occurs by weighted linear combination of all user I and Q components, after which the net I and Q channels are coherently upconverted 19 and combined into a QPSK spread waveform. Multplexing introduces another 4–5 dB of peak-to-average power variation.